Oriented silicon carbide sintered compact and method for preparation thereof

ABSTRACT

A novel oriented sintered silicon carbide product, wherein α silicon carbide is orientated in an arbitrary direction and crystal orientation is controlled to an arbitrary direction; and method for preparing the oriented sintered silicon carbide product which comprises mixing α silicon carbide particles with a solvent to form an α silicon carbide slurry, solidifying the slurry, molding the resultant solid, followed by sintering, wherein the solidification and molding are carried out in a magnetic field of 1 T or higher.

TECHNICAL FIELD

The present invention relates to an oriented sintered silicon carbideproduct and a method for preparation thereof. More specifically, thepresent invention relates to a novel oriented sintered silicon carbideproduct having a texture with the crystal orientation controlled, and amethod for preparation thereof.

BACKGROUND ART

A silicon carbide is a material having a high heat conductivity and aheat resistance, and furthermore, the strength, the hardness and thewear resistance so that it is used widely for a refractory, a polishingmaterial, a metallurgy material, or the like. For example, it is usedalso for the special applications such as a covering material for anuclear reactor particle fuel, and a lining material for a refuseincinerator.

Then, since it endures the chemical corrosion such as oxidizationcompared with the metals and the other non oxide based ceramics, it isused also in various kinds of crucibles, parts for baking in a furnace,a mechanical seal, a heat exchanging device heat transmitting tube, orthe like as a high-temperature structural material.

Moreover, in terms of the electro magnetic characteristics, it is usedas a heating element, and furthermore, its utilization as asemiconductor at a high temperature calls the attention owing to thelarge band gap so that the study and development thereof is promoted.

Concerning the silicon carbide utilized in the various fields, recently,the electric insulation property of a sintered silicon carbide with aberyllia addition has been found out so that a silicon carbide havingboth the high heat conductivity known conventionally and the electricinsulation property can be obtained. This beryllia-containing siliconcarbide with both the high heat conductivity and insulation has athermal expansion coefficient which is close to that of the silicon, sothat a silicon semiconductor chip can be mounted directly on thissilicon carbide, it has already used as a heat discharging substrate foran LSI and a laser diode, which is required to have the electricdischarging property and the electric insulation property at the sametime.

On the other hand, with regard to the ceramics including the siliconcarbide, in general, we can improve the properties such as the toughnessand the strength by controlling the microstructure thereof. Research anddevelopments have actively been conducted for providing the anisotropyto the properties or improving the properties by the crystallineorientation of the ceramics to a certain direction.

For example, as to the silicon carbide, obtainment of a silicon carbidewith the orientation properties and the excellent strength and toughnessby the zone sintering at the time of synthesizing the silicon carbideusing the reaction sintering is disclosed in the official gazette of theJapanese Patent Application Laid-Open (JP-A) No. 9-175870. However,according to this method, since the free Si is included, a problem isinvolved in that the high temperature strength of the silicon carbide tobe obtained is lowered. Moreover, a method of producing a siliconcarbide material having an oriented rod-like texture by orienting therod-like particle in a green compact to one direction by molding akneaded product comprising silicon carbide powders, a sintering aid andsilicon carbide rod-like particles by a molding method of applying thesharing force such as extrusion molding and a tape casting, andfurthermore, by orienting by the grain growth of the rod-like particlesas the seed crystals at the time of sintering is disclosed in theofficial gazette of JP-A No. 11-79845. However, according to the method,the shape of the silicon carbide to be obtained is limited due to themolding method of applying the shearing force to the kneaded product sothat it is disadvantageous in that the orientation direction cannot beset optionally. Furthermore, Michael D. Sacks, et al. reports theobtainment of an oriented silicon carbide by sintering the green compactprepared by cast molding of a silicon carbide slurry including aplate-like silicon carbide in J. Am. Ceram. Soc., 79[6] 1611 (1996).However, also according to this method, since the silicon carbide isoriented only in the specific direction of the cast molding direction, aproblem still remains in that the orientation direction is limited.Moreover, the orientation control using only the spherical particlescannot be executed by the conventional methods.

Accordingly, the present invention has been achieved in view of theabove-mentioned circumstances, and an object thereof is to provide anovel oriented sintered silicon carbide product having a texture withthe crystal orientation controlled, capable of solving theabove-mentioned problems in the conventional techniques, and a methodfor preparation thereof.

DISCLOSURE OF THE INVENTION

Accordingly, the present invention has been achieved in view of theabove-mentioned circumstances. It solves the problems of theconventional technique and provides the following inventions.

That is, firstly, the present invention provides an oriented sinteredsilicon carbide product characterized in being a sintered compact of anα silicon carbide with the orientation control.

Then, secondly, the present invention provides the above-mentionedoriented sintered silicon carbide product characterized in being asintered compact of an α silicon carbide with the orientation controlledby the application of a magnetic field.

Moreover, the present invention thirdly provides a method for preparingan oriented sintered silicon carbide product characterized in preparingan α silicon carbide slurry by mixing α silicon carbide particles and asolvent, solidifying and molding the slurry in a magnetic filed of 1 Tor higher, and sintering. Fourthly, it provides the method for preparingan oriented sintered silicon carbide product characterized in that theshape of the α silicon carbide particles is spherical.

The present inventors have already found out that the crystalorientation can be made by the application of the strong magnetic fieldat the time of producing oxide based non ferromagnetic ceramics such asan alumina, a titania and a zinc oxide (Patent Application No.2001-144049). Then, according to the invention, attention has been paidto the α-silicon carbide, of which magnetic anisotropy hasconventionally been dealt with as negligible due to its extremely smallmagnetic susceptibility so as to find out that the orientation controlof the silicon carbide by the strong magnetic field can be made withoutthe limitation to an oxide based non ferromagnetic material, which hasled to the present invention.

According to the conventional knowledge that there are an extremelylarge number of polymorphisms (2 H, 4 H, 6 H, 15 R, or the like) for thesame α silicon carbides in a state not simple but with a plurality ofpolymorphisms present in a mixed manner in many cases so as to bemodified during a sintering operation so that the orientation controlcan hardly been made, the present invention has not at all beenimaginable or predictable.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic illustration of the direction of both the slipcasting and the applied magnetic field in an example.

FIG. 2 shows the X-ray diffraction profiles of an oriented sinteredsilicon carbide product prepared by the example.

FIG. 3 show a schematic illustration of the direction of both the slipcasting and the applied magnetic field and the X-ray diffraction profileof an oriented sintered silicon carbide product prepared by thecomparative example.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention has the above-mentioned characteristics.Hereafter, embodiments thereof will be explained in detail.

First, a method for preparing an oriented sintered silicon carbideproduct provided by the present invention is characterized in preparingan α silicon carbide slurry by mixing α silicon carbide particles and asolvent, solidifying and molding the slurry in a magnetic filed of 1 Tor higher, and sintering.

The a silicon carbide as the material having a hexagonal system crystalstructure in material having an extremely small magnetic anisotropy. Itis preferable to use particles having about 1 mm or less size, as the asilicon carbide. Although it is possible to use a flat plate like one asthe α silicon carbide, since it is not necessary to use the plate likecrystal as the seed for the grain growth according to the presentinvention. Moreover, in the case of obtaining a silicon carbide withfine microstructure comprising equiaxial grains, it is preferable to usespherical particles. Of course, “spherical” here does not denotespherical in a strict sense, but it denotes those which can be regardedas spherical as a whole compared with plate like ones. It may beslightly angular microscopically.

As a solvent, a water or an alcohol such as non water based ethanol, anorganic solvent such as an ether, a solvent mixture thereof, or the likecan be used.

By mixing the above-mentioned α silicon carbide particle and solvent, aslurry is prepared. The slurry concentration and the electrolyteconcentration can be determined in consideration of the particle size ofthe α silicon carbide, the kind of the solvent, the molding method inthe next process, or the like. Moreover, various kinds of sinteringaids, or the like can be added to the slurry. As to the slurry pH inthis case, pH 9 or higher can be considered as the general standard. Forexample, for a water based slurry, it is in a range of about pH 9 to 12.

Next, the slurry is solidified and molded in a magnetic field. Anintensity of a certain degree is required to the magnetic field fororienting an α silicon carbide having a small magnetic anisotropy. Anstrong magnetic field can be obtained relatively easily without using aliquid helium owing to the recent development of a superconductivemagnet. Even in the case of a non magnetic material, the magnetic fieldcan be applied as an unnegligible energy applied from the outside. Asthe standard of such a magnetic field, it is 1 T or higher and it ismore preferably 5 T or higher, such as about 10 T. Moreover, as to themagnetic field applying direction, an optional direction can beselected. Without being limited by the direction or shape of molding, anorientation corresponding to the application of a magnetic field can beadopted.

Furthermore, for molding, various kinds of methods can be utilized. Forexample, colloid process such as slip casting, gel casting, pressurefiltration, tape casting, and electrophoresis deposition, or the likecan be presented as the preferable methods.

Then, by sintering the obtained compact, an oriented silicon carbide canbe obtained. For the sintering operation, various sintering methods andsintering conditions can be considered according to the shape, themicrostructure, or the like of the desired oriented silicon carbide tobe obtained. A sintering aid can be used as well. For example, as to thesintering conditions, although it differs depending on the sinteringaid, the temperature range of 1,800 to 2,300° C. in an inert gasatmosphere for about 1 to 3 hour duration can be presented as anexample. According to the sintering operation, the densification of themicrostructure and the orientation can be promoted in the grain growthprocess.

An oriented silicon carbide of the present invention obtainedaccordingly has the α silicon carbide having the orientation control inan optional direction, with the strength and the toughness improved atthe same time, and furthermore, the heat conductivity improveddramatically. Moreover, since the oriented silicon carbide having finemicrostructure comprising equiaxial grains can be provided, the strengthand the toughness can further be improved so that the orientationdependency of these properties can be reduced. Furthermore, since thegrain growth in a specific direction is restrained, the stableproperties can be obtained even at a high temperature.

Hereinafter, examples will be presented with reference to theaccompanied drawings for explaining the embodiments of the presentinvention in further details.

EXAMPLE Example

A slurry was produced by mixing 4.3 weight part of an alumina having a0.5 μm average particle size as a sintering aid in 100 weight part ofα-silicon carbide powders having a 0.5 μm average particle size, anddispersing the same in an aqueous solution measured so as to have a 30vol % solid phase concentration, and adjusted to pH 10. In order todisperse the particles weakly aggregated at the time, ultrasonicagitation was carried out while dispersing with a stirrer. An operationof pouring the slurry into a porous mold for absorbing the solution forthe high density molding (slip casting) was executed in a magnetic fieldof 10 T. As shown in FIG. 1, the direction of the applied magnetic fieldwas parallel to the casting direction. By heating the compact at 1,950°C. for 2 hours in an argon atmosphere, an oriented sintered siliconcarbide product was obtained. FIG. 2 shows the results of the X raydiffraction measurement of the obtained oriented sintered siliconcarbide product.

From FIG. 2, it was confirmed that an oriented sintered silicon carbideproduct with the 2 H (200) plane and the 6 H (102) plane oriented neatlyto the plane (T) perpendicular to the magnetic field applying directionand the (100) plane to the parallel plane (S) was obtained.

Comparative Example

A compact was produced by slip casting of a slurry obtained in the samemanner as in the example without applying the magnetic field. A sinteredsilicon carbide product was obtained by heating the compact at 1,950° C.for 2 hours in an argon atmosphere. FIG. 3 shows the results of the Xray diffraction measurement of the obtained oriented sintered siliconcarbide product.

From FIG. 3, it was confirmed that the similar diffraction lines wereobserved both in the plane (T) perpendicular to the magnetic fieldapplying direction and the plane (S) parallel thereto so that thesilicon carbide was not oriented in the obtained sintered compact.

Or course the present invention is not limited to the examples mentionedabove, and it is needless to say that various embodiments can beprovided in the details.

INDUSTRIAL APPLICABILITY

As heretofore explained in detail, according to the present invention, anovel oriented sintered silicon carbide product with the crystalorientation in an optional direction, having a minute texture comprisingisometric grains, and a production method therfor are provided.

1-4. (canceled)
 5. An oriented sintered silicon carbide product, being a sintered compact consisting primarily of an α silicon carbide with the orientation controlled by the application of a magnetic field.
 6. A method for preparing an oriented sintered silicon carbide product which comprises preparing an α silicon carbide slurry by mixing α silicon carbide particles and a solvent, solidifying and molding the slurry in a magnetic filed of 1 T or higher, and sintering.
 7. The method for preparing an oriented sintered silicon carbide product according to claim 6, wherein the shape of the α silicon carbide particles is spherical. 